In the field of organic semiconductors which utilizes an organic compound with a carrier mobility, application of a voltage across the organic semiconductor thin-film increases carrier density, which permits flow of an electrical current between a pair of electrodes provided on an organic semiconductor thin-film. Thus, researches have been conducted on organic transistors, which permits a gradual utilization of the organic semiconductors in technical fields such as transmitting, processing, and recording and displaying of information. In such technical fields, an electrical signal is used for controlling electron carriers and hole carriers within the organic semiconductor at a junction interface between a metal and an organic semiconductor or between one organic semiconductor and another organic semiconductor.
For example, there is a structure of an organic MOS-TFT using an organic semiconductor thin-film in which a gate electrode, a gate insulating layer, a source electrode and a drain electrode, and an organic semiconductor layer are formed on a substrate. Since an enhancement of the carrier mobility is necessary to increase a working frequency of the organic transistor, a layered structure comprising the organic semiconductor layer by using pentacene or the like, and the gate insulating layer by using PMMA, cyclohexene or the like is proposed. Materials used for the electrodes are Pd, Au and the like for both the source electrode and the drain electrode.
In convention, the same material is commonly used for the source and drain electrodes of the p-type and n-type channel organic semiconductor elements of complementary organic semiconductor devices (for example, see Bell Lab. Nature Vol. 403, 521, 2000).
Such conventional organic semiconductor device has a drawback such that the right materials have not been put in the right places of the p-type and n-type channel organic semiconductor elements concerning the work function, a sufficient ohmic contact is not obtained between the organic semiconductor layer and each electrode, so that there are an increase of driving voltage for the organic semiconductor element and a nonlinear current-voltage characteristics.